1. Field of the Invention
The invention relates to a control method and a driving system, and more particularly to a control method and a driving system adapted for a light emitting device.
2. Description of the Related Art
Brightness of light emitted by an LED (light emitting diode) device is controlled by an LED driving system providing a constant current to the LED device for different periods of time, where the constant current refers to a constant current value within a unit time period.
Referring to FIGS. 1 and 3, a conventional LED driving system has 16 driving channels to drive the LED device (not shown), and receives 16 sets of source logic data respectively corresponding to the 16 driving channels. Each set of source logic data is composed of 6 brightness bits to indicate one of 26 levels of brightness. The brightness bits have different bit orders defined to be 0 to 5, and are called 0th to 5th brightness bits herein. The LED driving system divides the source logic data into 6 sets of logic data, each of which has 16 logic values respectively for the 16 driving channels and corresponds to a respective one of the brightness bits. The LED driving system includes a control unit 10, a shift register unit 11, a data latch unit 12 and a driving unit 13.
The control unit 10 receives the source logic data, and is configured to generate the logic data after division, a clock signal, a latch signal and an output enable signal.
The shift register unit 11 includes 16 registers, receives the clock signal and the logic data, and sequentially and respectively stores the logic values in the registers in response to a positive edge of the clock signal.
Further referring to FIG. 2, the control unit 10 enables the shift register unit 11 to store the 6 sets of logic data corresponding to the brightness bits having the bit orders 0 to 5 (referring to numbers shown in the logic data in FIG. 2) in the given sequence. A length of time required by the shift register unit 11 to store each set of logic data is T1.
The data latch unit 12 includes 16 latches, receives the latch signal, and respectively stores into the latches the logic values stored in the shift register unit 11 in response to a positive edge of the latch signal.
The driving unit 13 receives the output enable signal and the logic values stored in the data latch unit 12, and outputs, to each of the driving channels, a constant current signal for one of six predetermined time periods. Further referring to FIG. 2, each of the predetermined time periods has a length of 2kT2 according to the output enable signal and the logic data, where k represents the bit order of the brightness bit corresponding to the logic data received thereby, and T2 is a length of the predetermined time period corresponding to the brightness bit having the bit order of 0. In an example, when both of the output enable signal and the corresponding logic value has high logic levels, the corresponding channel outputs a first constant current to the corresponding LED, and when the output enable signal has the high logic level and the corresponding logic value has a low logic level, the corresponding channel outputs a second constant current (e.g., having a magnitude of 0A) to the corresponding LED.
In this configuration, when 2kT2<T1, there is a time period toff in which the LED device is in an idle state, thereby limiting a utilization rate and maximum brightness of the LED device. When 2kT2>T1, there is a time period Doff in which control unit 10 is unable to output the next set of logic data that corresponds to the brightness bit having the bit order of (k+1), thereby limiting a refresh rate of the LED device.